(1) Field of the Invention
The present invention relates to integrated operational amplifiers and, more particularly, to analog, wide temperature range, quad operational-amplifiers.
(2) Background of the Invention
Electronic devices permeate the modern world. They are used in everything from appliances to computers to state-of-the-art scientific apparatuses. Given the broad range of applications and, accordingly, the broad range of environments in which modern electronics are required to reliably operate, means for hardening electronic devices against extreme environmental variables.
Perhaps the harshest environment in which modem electronics are required to operate is outer space. In a typical space application, when a circuit is exposed to direct sunlight, its temperature can rise to over 120° C., higher than the boiling temperature of water on Earth. In the absence of sunlight, the vacuum of space can rapidly cool the same circuit to 180° C. below zero, a temperature nearly cold enough to make liquid Nitrogen on Earth.
Modern integrated circuit elements require current references to accomplish proper circuit element biasing. If the same circuits are to operate in extreme temperatures, these circuits require current references that stay nearly constant over the required operating range, otherwise circuit performance will change when integrated transistors become improperly biased.
In the best-case scenario, improper biasing causes changes in the transistors' inversion levels, electron and hole mobilities, and thermal voltage levels. These changes drastically alter the performance characteristics of transistors and typically render a circuit useless unless care has been taken to compensate for such changes. In the worst-case scenario, changes in the current reference lead to overvoltage or voltage surges in the circuit, causing irreversible failure of the device; the mechanisms for such non-reversible failure are typically hot carrier injection and breakdown of oxide layers due to high gate-to-source voltages.
In the past, attempts to make current references that are resistant to such temperature effects have primarily used one of two transistor integration techniques: the first is the constant transconductance method and the second is the constant current method.
The constant transconductance method minimizes variations in small-signal performance parameters, such as bandwidth, when temperatures change. However, the reduction of temperature sensitivity to small-signal parameters leads to increased temperature sensitivity in large-signal parameters, such as slew rate.
As an alternative to the constant transconductance method, the constant current method has been used. The constant current method minimizes variations in large-signal parameters at the expense of small-signal parameters.
In general, it is desirable to simultaneously minimize variations in both small-signal parameters and large-signal parameters with respect to temperature, and neither of the above-described methods can achieve simultaneous minimization of both small-signal and large-signal parameter variations with temperature.
Another extreme environmental variable to which circuits can be exposed is radiation. Radiation degrades transistor performance by knocking atoms out of lattice sites, which cause defects, and scattering electrons and holes out of regions to which they would otherwise be bound. Both processes drastically change the functionality and characteristics of transistors.
Transistors can be exposed to radiation in many different environments, non-limiting examples of which include instrumentation for nuclear reactors, modern laboratory environments in which radioisotopes or cosmic background radiation are studied, and outer space.
Typical metal-on-silicon (MOS) components do not withstand radiation, making the devices unreliable in applications in which they are exposed to radiation. However, several modern technologies, for example silicon-on-insulator (SOI) MOS components, are more resistant to radiation damage than typical bulk MOS devices. SOI MOS devices, as an example, are surrounded by insulator layers, non-limiting examples of which are silicon nitride and silicon oxynitride, that shield the silicon and metals from radiation.
Finally, as discussed above, overvoltage can typically cause non-reversible damage to integrated circuit components. Exacerbating such problems is the recent move of the state-of-the-art from 5 Volt transistors to 3.3 Volt transistors. If new circuits using the 3.3 Volt technology are to be integrated with old circuits and power supplies using the 5 Volt technology, steps must be taken to ensure that overvoltage events do not destroy circuits.